1. Field of the Invention
The subject invention generally pertains to heat exchanger wheels and more specifically to a pair of heat exchanger wheels that are driven at varying speed to meet a varying cooling and dehumidification demand.
2. Description of Related Art
A comfort zone, such as one or more rooms or an area within a building, is often cooled by a refrigeration system. A typical refrigeration system includes a compressor, a condenser, a flow restriction (e.g., an expansion valve, orifice, etc.), and an evaporator connected in series flow relationship with each other to comprise a closed loop refrigerant-filled circuit. Depending on the arrangement of the system's components, many such systems can be used for both heating and/or cooling. When used for cooling, the evaporator absorbs heat from the comfort zone, while the condenser expels waste heat to atmosphere. Cool supply air for cooling the building can be provided by passing warmer air (e.g., outdoor air, indoor air, or a mixture thereof) directly across the refrigerant-filled evaporator before discharging it into the building. In some systems, the evaporator cools the supply air indirectly by first direct cooling water. The chilled water is then circulated through another heat exchanger, which in turn cools the supply air. Both direct cooling systems and chilled water systems are used in cooling commercial buildings and often take the form of a rooftop unit.
With a rooftop unit, the refrigeration system is primarily contained within a sheetmetal housing installed on the roof of a building. Ductwork passing through the roof conveys cool supply air and return air between the housing and the comfort zone of the building. When the zone calls for cooling, a thermostat signals the refrigeration system to start the compressor, a supply air blower, and possibly a chilled water circulation pump, if used. The blower forcing cool supply air into the comfort zone displaces warmer return air that is exhausted to atmosphere, or sometimes part or all of the return air is recirculated, i.e., recooled and returned to the comfort zone as supply air. When the thermostat indicates that the cooling demand has been met, the refrigeration system typically shuts off. This cycle is repeated as frequently as needed to meet the cooling demand.
When the cooling demand is relatively low in comparison to the cooling capacity of the system, the system only runs briefly between cycles. Unfortunately, such short cycling of the system is especially hard on a compressor and may shorten its life. This is a common problem, as refrigeration systems are generally sized to handle the largest anticipated cooling load of the building. Such an approach to sizing a refrigeration system also tends to be more costly than choosing a system that more closely matches the overall cooling needs.
Although, a refrigeration system typically turns off upon bringing the temperature of the room back down to a set point, in some instances, the humidity of the room may still be uncomfortably high: leaving the room feeling cold and dank. In such cases, the refrigeration system may be run a little longer just to bring the humidity down. However, that can lower the room temperature to an uncomfortable level. So various other methods are used to reduce the humidity.
For example, the supply air can be directed through a drying wheel containing a desiccant (e.g., calcium chloride, lithium chloride, zeolite, etc.) that absorbs moisture from the air. Subsequently, the desiccant is heated to drive the moisture from the desiccant, so the wheel can take on more moisture from the air. The moisture absorbing and drying cycles are typically carried out at the same time at opposite halves of the wheel as the wheel turns. An external heat source (e.g., gas or electric heat) dries one half of the wheel, while supply air passes through the other half. Primary drawbacks of such a system is the initial cost of a desiccant filled wheel and the ongoing energy costs of adding heat to a system whose primary function is cooling.
Another dehumidification system involves a reheat coil downstream of the cooling coil. The cooling coil brings the temperature of the supply air below its dew point to condense moisture from the air. The reheat coil subsequently raises the supply air temperature, so that the comfort zone is not cooled excessively. In some cases, a refrigeration system's condenser can serve as the reheat coil. However, just as with a desiccant system, adding such heat to a cooling system can be inefficient and costly.
In some instances, high humidity can lead to freeze-up of the cooling coil. For example, condensation from incoming air passing through the cooling coil may freeze to the outer surface of the coil. Ice accumulating on the coil obstructs the supply airflow, which reduces the load on the coil. This in turn promotes further buildup of ice.
To prevent freeze-up, the refrigeration system can be periodically turned off, but that can lead to short cycling and all of its related problems. Another solution is to provide a defrost cycle where heat is applied to the coil. However, there are obvious disadvantages of adding heat to a cooling system, as previously explained.